Hello, I have been looking through some examples and resources both here and anywhere I can find and I am not quite sure why my code does not appear to work.

I am trying to test allocating DPS memory and writing to it (just 0's for now) but every time I allocate the page and then write to it I get a general protection fault. Looking at the DPMI 1.0 spec it says I am not supposed to modify any LDT information for DOS allocations, so I avoided that.

IO am using CWSDPMI and the code is as follows:



The general protection fault I have found happens exactly at the rep stosb line. I must be missing something?
Note: If there are typos in the code it is likely because I retyped all of this looking at my other computer screen.

DX is the pmode selector
AX is the real mode segment

The offset should be 0

Ah that is actually making more sense now, I'm more of a 65xx / 68xx kind of person so I'm half confused about protected mode selectors and real mode segments, etc. I'm assuming it's not as simple as throwing ax into es and 0 into di and using stosb from there, right? that causes a protection fault as well. Do I have to convert it into a physical address or something somehow and go from there? By the nature of what I read about how stosb works, it doesn't seem that way. I'd hate to ask but I would definitely appreciate an example that I can study of how this would work in particular with function 0x100 like used here (a couple examples I looked at online to accomplish certain tasks used it), so that I can learn a little more about that aspect.

When you are using that memory, you are in protected mode and would use DX:0 to access it.
Usually you would allocate DOS memory to use with a real mode DOS or BIOS call, in that case you pass AX:0 as the memory address.